This project involved studies of modification of genomic size of viral RNA and transcription of viral specific sequences in mouse S plus L minus revertants. A summary of the major findings is as follows: (1) the modification of the genomic sizes of both murine sarcoma and leukemia viruses was detected during co-infection of both viruses; (2) in spontaneous or chemically induced S plus L minus mouse revertants, the same portion of sequences common to both sarcoma and leukemia viruses was detected; (3) Gazdar murine sarcoma virus (Gz-MSV) is closely related to Moloney murine sarcoma viruse (M-MSV) and Gz-MSV is possibly a variant of M-MSV.